A process for producing potassium hexatitanate fibers having a tunnel structure, which comprises mixing a titanium component represented by the general formula (Ti, M)O.sub.2 where M is metal impurities with a potassium component selected from the group consisting of potassium oxide, potassium compounds capable of forming potassium oxide when heated and a mixture thereof, to obtain a mixture represented by the general formula K.sub.2 O.n(Ti, N)O.sub.2 where n is from 1.5 to 2.5 and M is as defined above; heating the mixture to obtain a melt; forming from the melt a fibrous material having a layered crystalline structure corresponding to potassium dititanate (K.sub.2 O.2TiO.sub.2); leaching a part of K.sub.2 O from the fibrous material; and heating the fibrous material at a temperature of at least 800.degree. C. and lower than the melting point of the fibrous material.